1. Field of the Invention
This invention relates to a fluorescent display device which is adapted to control electrons emitted from cathodes by means of control electrodes and selectively impinge them on anodes arranged on a substrate to effect luminous display, and more particularly to a fluorescent display device of which luminous display is observed without being hindered by the cathodes.
2. Description of the Prior Art
A fluorescent display device is typically constructed in such a manner as shown in FIG. 1. More specifically, it includes a substrate 1 formed of an insulating material such as glass or the like, and a plurality of anodes 2 arranged on the substrate 1 and each comprising an anode conductor 3 and a phosphor layer 4 deposited thereon. The apparatus also includes control electrodes 5 arranged at a certain distance from the anodes 2 and filamentary cathodes 6 stretched at a distance further away from the anodes 2. Reference numeral 7 indicates a cover plate disposed opposite to the substrate 1 and 8 designates wiring conductors.
The conventional fluorescent display device constructed as described above has a disadvantage that the filamentary cathodes 6 hinders luminous display of the anodes 2 from being observed through the cover plate 7 to deteriorate the visibility of the device, because the cathodes are interposed between the anodes 2 and the cover plate 7 opposite thereto.
This disadvantage is encountered with not only a fluorescent display device of the front emission type that the cover plate 7 is formed of a light-permeable material to cause luminous display of the anodes 2 to be observed through the cover plate 7 or from the lower side in FIG. 1, but a fluorescent display device of the type that luminous display is observed through the substrate 1 or the upper side in FIG. 1, because the red-hot filamentary cathodes 6 are intermittently observed through interstices each formed between each adjacent two anodes 2.
A fluorescent display device has been extensively used for various applications, for example, for a variety of display apparatus mounted on vehicles such as a car and the like. A fluorescent display device used for vehicles generally undergoes a specific restriction that only a single power source (a battery of 12 V or 24 V) is used for its power source. In general, in a fluorescent display device, it is required to keep potential of cathodes substantially lower than that of a power source in order to utilize potential between anodes and the cathodes. For this purpose, it is necessary to cause voltage drop across the power source to be generated by a suitable means such as a resistive divider to maintain the cathodes at suitable potential. However, the single power source has a serious problem of causing much power loss when the voltage drop occurs, because a large amount of current must be flowed through the filamentary cathodes to heat them.
In view of the foregoing, an indirectly heated cathode structure has been proposed which comprises electron emitting cathodes and heaters for heating the cathodes arranged in a manner to be electrically separate from each other and is adapted to keep them at different potentials. In the indirectly heated cathode structure, voltage applied to the heaters for heating the cathodes is not substantially limited. Accordingly, the direct connection of the heaters to a power supply may be permitted. This avoids dropping voltage across the power source by means of a resistive divider or the like to obtain lower voltage, resulting in the heaters effectively utilizing electric power. Further, the indirectly heated cathode structure has another advantage that the cathodes are merely required to be earthed, thereby power consumption for ensuring cathode potential required may be eliminated.
However, the indirectly heated cathode structure is obliged to have an highly increased diameter, because it is so constructed that an electrically conductive core is coated with an insulating layer, on the outer surface of which cathodes are then provided. Accordingly, the application of the indirectly heated cathode structure to a fluorescent display device used for a vehicle further promotes the above-described deterioration of visibility of the fluorescent display device.
Furthermore, the use of sulfide phosphor such as ZnS:Cu, (Zn,Cd)S:Ag or the like as often observed in a multi-color fluorescent display device leads to the generation of S gas and/or sulfide gas such as SO.sub.2 and the like from the phosphor due to the impingement of electrons thereon, which substantially deteriorate an electron emitting material, such as, for example, (Ba,Sr,Ca)O contained in the cathodes to significantly decrease the electron emitting capability of the cathodes, resulting in the lifetime of the fluorescent display device being highly reduced.